This invention relates to an optical element for use in cables. The optical cable element includes a coated optical waveguide fiber loosely contained in a tube whose nominal outer diameter may be 900 .mu.m.
Optical waveguide fibers, herein called optical fibers, typically comprise a glass core and a cladding. The core normally has an index of refraction which is greater than that of the cladding. During manufacturing, the cladding is typically coated with one or more layers of a thin plastic material, such as a UV-curable acrylate polymer. As used herein, this initial protective layer or layers will be referred to collectively as the fiber's "first protective coating". Typical outside diameters for an optical fiber are approximately 8 .mu.m for a single mode core (or 50-62.5 .mu.m for a multimode core), 125 .mu.m for the cladding, and 250 .mu.m for the first protective coating. For example, Corning's SMF-28.TM. CPC6 single-mode optical fiber, optimized for use in the 1310 nm wavelength region, has a core diameter of 8.3 .mu.m, a cladding diameter of 125 .mu.m, and a first protective coating diameter of 245.+-.10 .mu.m. Optical fibers having a first protective coating will be referred to as "coated optical fibers".
Coated optical fibers may be used in telephony, cable television, marine, and private network applications for the transmission of data, voice, and/or video.
Coated optical fibers usually are colored and given additional protection before they are put to use in a particular application. "900 .mu.m buffered fiber" is the term used for a product having a standard size outer diameter of 900 .mu.m and including a single coated optical fiber. For example, Siecor's 900 .mu.m OptiStrip buffered fiber includes a coated optical fiber having an outer diameter of 250 .mu.m which is surrounded by a layer of loose aramid fiber yarn. The layer of aramid fiber yarn is contained within a tube formed of plastic material and having an outer diameter of 900 .mu.m. This product has a specified short term maximum tensile load of 6 N, a specified long term maximum tensile load of 3 N, a specified short term minimum bend radius of 5.0 cm, and a specified long term minimum bend radius of 3.0 cm.
A single 900 .mu.m buffered fiber may be incorporated into a single fiber cable. Cables including 900 .mu.m buffered fibers typically are designed for indoor use. For example, in Siecor's single-fiber OptiStrip cable, the Siecor 900 .mu.m OptiStrip buffered fiber is surrounded by a second layer of aramid fiber yarn which is contained within an outer jacket formed of plastic material. This product has an outer diameter of 2.9 mm and a weight of 7 kg/km. The product complies with Bellcore specification TR-NWT-000409, and has the following mechanical and environmental specifications:
______________________________________ Maximum tensile load (short term, long 220 N, 88 N term) Minimum bend radius (short term, long 5.0 cm, 3.0 cm term) Crush resistance 3.5 N/mm Impact resistance 20 cycles Cyclic flex resistance 100 cycles Maximum vertical rise 1000 m Storage temperature -40 to +70.degree. C. Operating Temperature 0 to +50.degree. C. ______________________________________
Other optical cables include a plurality of 900 .mu.m buffered fibers. Examples are Siecor's MIC and UMIC cables. These cables have not met the -20.degree. C. low operating temperature objective set by Bellcore TR-409. The low operating temperature for these cables has been -10.degree. C.
In optical fiber products, a layer of aramid fiber yarn typically acts primarily as a tensile strength member. The layer of aramid fiber provided in OptiStrip buffered fiber is not designed to serve as a tensile strength member; this is reflected by its low maximum tensile load specification. Instead, the layer of aramid fiber yarn has been provided for cable processing considerations. Specifically, the layer of aramid fiber acts to separate the coated optical fiber from the tube having an outer diameter of 900 .mu.m. If the coated optical fiber outer surface contacts the tube, they have tended to stick to each other. This sticking tends to degrade low temperature performance, as the coated optical fiber may well have a coefficient of thermal expansion different from that of the tube. The aramid yarn layer also tends to improve strippability of the product.
The addition of the aramid fiber yarn layer has some disadvantages of its own, however, in that it adds to the cost of the product, and adds an additional yarn application step. Sticking has not presented a problem for coated optical fibers if the space between the coated optical fibers and the tube is sufficiently great. Siemens Telecom Report Vol. 2, No. 3, September 1979, on page 169 presents an optical fiber cable including buffer tubes having an outer diameter of about 1.4 mm enclosing coated optical fibers having an outer diameter of about 0.14 mm. No aramid yarns or other fillers are inserted between the coated optical fiber and the buffer tube.
A problem in cables having no fillers and less free space between the coated optical fibers and the enclosing protective tube is how to omit the aramid yarn layer, which is not recommended for use during cable pulling, without degradation of a cable's low temperature performance or strippability. 900 .mu.m (0.9 mm) is a smaller outer diameter than 1.4 mm, and this problem exists in particular for cables containing 900 .mu.m buffered fibers.